Aramaic was probably the language that Jesus spoke, but the texts of the New Testament were all written at least a generation after Jesus' death, and all are in Greek. It's called Koinê Greek, or "common" Greek, because it was the lingua franca of the Mediterranean after the conquests of Alexander the Great (as Aramaic had been the lingua franca of the Assyrian Empire). We have other authors who wrote in Koinê (Epictetus, Lucian) and it's relatively easy to read.

The earliest texts that were authorized as Scripture in the 4th century CE were the letters of Paul, which constitute 13 of the 27 books of the NT. None are firmly dated, but he was probably writing the earliest ones in the early to mid-50s CE. He was writing to "Gentile" converts to Christianity in Greek speaking cities, for the most part. The Gospels were later.

Thanks for reading and responding, my friend.